The present invention relates generally to gas turbine engines, and, more specifically, to turbine cooling therein.
In a gas turbine engine, air is pressurized in a compressor and mixed with fuel in a combustor for generating hot combustion gases. Energy is extracted from the combustion gases in turbine stages which power the compressor through one drive shaft, and produce additional work for powering an upstream fan in a turbofan aircraft engine application, or driving an external drive shaft for marine and industrial (M&I) applications.
The basic core engine typically includes a multistage axial compressor having rows of compressor blades and corresponding guide vanes which pressurize ambient air in stages and correspondingly increase the temperature thereof. The air discharged from the aft end of the compressor has the highest pressure, commonly referred to as compressor discharge pressure (CDP), and a correspondingly high temperature.
In an exemplary configuration, the compressor may have seven stages for increasing air pressure many times atmospheric pressure along with many hundreds of degrees of temperature increase due to the compression cycle. A fewer or greater number of compression stages may be used as desired for the specific design of the gas turbine engine and its intended use.
A majority of the CDP air discharged from the compressor is mixed with fuel in the combustor for generating hot combustion gases. These combustion gases then undergo an expansion cycle in the several turbine stages for extracting energy therefrom which correspondingly reduces the pressure of the combustion gases and the temperature thereof. A high pressure turbine (HPT) immediately follows the combustor and is used to power the compressor blades in the core engine.
A low pressure turbine (LPT) follows the HPT and drives the second shaft for powering the upstream fan in the turbofan engine application, or driving an external drive shaft for M&I applications.
The overall efficiency of the gas turbine engine is dependent on the efficiency of air compression, efficiency of combustion, and efficiency of combustion gas expansion in the turbine stages.
Since turbine components are directly exposed to hot combustion gases during operation, they require suitable cooling thereof to ensure long useful life. For example, some of the compressor discharge air is diverted from the combustion process for cooling liners of the combustor itself, as well as cooling various components of the HPT.
Each turbine stage typically includes an upstream turbine nozzle or stator having a row of nozzle vanes which direct the combustion gases downstream through a corresponding row of turbine rotor blades. The blades are typically mounted to the perimeter of a supporting rotor disk in corresponding dovetail slots formed therein.
The turbine blades and vanes are typically hollow airfoils with corresponding internal cooling channels therein which receive compressor discharge air for cooling thereof during operation. The hollow blades and vanes typically include various rows of film cooling and other discharge holes through the pressure and suction sidewalls thereof for discharging the spent internal cooling air in corresponding external films for further protecting the airfoils.
Furthermore, the turbine rotor disk which supports the first stage turbine blades is a relatively large component with a rim in which the blades are mounted, a narrow web extending radially inwardly therefrom which terminates in a wider hub having a central bore. The rotor disk is subject to both considerable centrifugal loads and heating during operation and must also be designed for long life.
In contrast, the LPT requires less cooling than the HPT in view of the reduction in combustion gas temperature and pressure during the expansion cycle. The cooling requirements are therefore reduced, and typically interstage bleed air may be used for cooling various components therein.
The main turbine flowpath is designed to confine the combustion gases as they flow through the engine and decrease in temperature and pressure from the combustor. The various cooling circuits for the turbine components are independent from the main flowpath and must be provided with cooling air at sufficient pressure to prevent ingestion of the hot combustion gases therein during operation.
For example, suitable rotary seals are provided between the stationary turbine nozzles and the rotating turbine blades to prevent ingestion or back flow of the hot combustion gases into the cooling circuits. Since the airfoils of the nozzle vanes and turbine blades typically include rows of cooling air outlet holes, the cooling air must have sufficient pressure greater than that of the external combustion gases to provide a suitable backflow margin to prevent ingestion of the hot combustion gases into the turbine airfoils themselves.
Accordingly, the components of the HPT are typically cooled using the full-pressure CDP air, whereas LPT components may be cooled using lower pressure, interstage bleed air.
In this way, the use of compressor air for cooling turbine components may be matched to the different cooling requirements of the HPT and the LPT for reducing the use thereof and therefore increasing engine efficiency.
However, increasing engine efficiency is a continuing and paramount design objective in modem gas turbine engines, and it is therefore desired to further increase engine efficiency by reducing pressurized air extracted from the compressor.